The invention is useful in the production of continuous webs of wet fibrous materials, especially webs of cellulosic fibers used in the production of paper products, such as paper and paperboard. The drying of such webs is extremely important in the production of quality papers which should have as uniform moisture content as possible.
U.S. Pat. No. 3,864,842 relates to a multicylinder dryer section of a paper machine, and discloses numerous defects which can occur as a result of the non-uniform drying of a web of paper. This patent is typical of the many patents which are directed to the profiling of heat transversely of a traveling web by means of differently heated cylinders or streams of air to more uniformly dry the web and eliminate wet or dry streaks in the web, i.e. streaks which extend in the machine direction or longitudinally of the web and wherein the moisture content is appreciably different from portions of the web adjacent the streaks.
U.S. Pat. No. 3,948,721 relates the problems of dry streaking in greater detail, and describes how atomized sprays of water are utilized at the discharge end of the dryer section to wet dry streaks which appear in the web, so that the final product will have a more uniform moisture content. It was found that this particular solution to the problem occasionally produced other undesirable side effects in the wetted areas of the paper; namely, unsightly water marks, wrinkles, and blackening of the paper during the calendering operation. The invention is directed to an improved method of adding water to the web to eliminate streaks in the web without experiencing any of the aforementioned side effects.
The term "dry end", as used herein and in the claims, means a point along the pathway or line which the web travels during processing, where the moisture content of the web is 20 percent or less, by weight, whereas the term "wet end" means a point along the processing pathway or line where the moisture content of the web is at least 25 percent.
Briefly stated, the invention is in a method and apparatus used in the production of a continuous web composed of fibrous material, such as wood pulp used in the formation of a sheet of paper or paperboard. The method comprises the formation of wet fibrous material into a continuous web having a high moisture content which is reduced to about 50-70 percent, by weight, by passing the web through a press section, after which the web is guided through a dryer section, wherein the web is heated to reduce the moisture content even further to a desired level, e.g. below 10 percent, as it moves longitudinally along a pathway or processing line through the dryer section. The web is constantly monitored at the dry end of the processing line for dry streaks. In response to the sensing of a dry streak, a water spray that is upstream in the wet end of the processing line and longitudinally aligned with the dry streak sensed, is operated to accordingly wet the adjacent correlated area of the web to eliminate further dry streaks downstream in the dry end and consequently produce a web having a more nearly uniform moisture content.
An apparatus, designed to carry out the aforementioned process, comprises, for example, a series of heated cylinders which define the processing line, and associated felts for pressing the web against the heated cylinders. A sensor is located at the dry end of the processing line, usually adjacent the discharge end of the dryer section, whereas the sprays, responsive to the sensor, are positioned in the wet end of the processing line usually adjacent the charging end of the dryer section. The sprays are positioned to spray water directly against areas of the web correlated to the dry streaks requiring additional moisture, or against sections of a dryer felt which, in turn, contacts the correlated area of the web requiring wetting.
Another aspect of the invention is the provision of monitors in the dry end of the processing line to sense both dry and wet streaks. Similar correlated sprays and steam nozzles are located in the press section, just prior to the last press, to spray water or steam against appropriate sections of a felt that contact correlated areas of the web needing wetting or drying, prior to passage of the web through the last press of the press section. Water from the sprays cool the felt which, in turn, raises the viscosity of the water in the web which the felt contacts to subsequently reduce the amount of water removal in the press section, whereas steam from the nozzles has just the opposite effect, i.e. the steam heats the felt which lowers the viscosity of the water in the web which the felt contacts to improve the removal of water from the web in the press section. Thus, both the wetness and dryness of the web can be alternately controlled by the application of water and steam in the wet end of the processing line.